1. Field of the Invention
The present invention relates to an electroluminescent device (EL device) using an organic film containing an organic dye and, more particularly, to an organic EL device which is capable of high-efficiency light emission by combining a plurality of organic films.
2. Description of the Related Art
In recent years, an organic EL device used for, e.g., a display or illumination device has been extensively studied and developed. For example, Shogo Saito at Kyushu University reported an EL device with a two-layered structure of organic films using a metal electrode/aromatic dye/polythiophene/transparent electrode (J. J. Appl. Phys. 25, L773, 1986). In this report, the thickness of the organic film is 1 .mu.m or more, and an application voltage is as large as 100 V. In addition, C. W. Tang et al. of Kodak Corp. reported an EL device with a two-layered structure of organic films, i.e., Mg.Ag/Alq3/diamine/ITO (Appl. Phys. Lett., 51, p913, 1987). According to this report, an EL device driven by an application voltage of 10 V or less to exhibit a sufficient luminance for practical use can be obtained by setting the thickness of the organic film to be 1,000 .ANG. or less. Each of these EL devices basically has an organic two-layered structure obtained by combining electron injection type and hole injection type dyes. The principal characteristics of these EL devices are that the thickness of the organic film is decreased as small as possible, that a material having a small work function is selected as a metal electrode on the electron injection side, that an organic material for preventing an electrical defect when an organic film is formed by vacuum evaporation or sublimation is selected, and the like. Shogo Saito at Kyushu University also proposed a device with an organic three-layered structure, i.e., an electron-injection layer/a light-emitting layer/a hole-injection layer. In this structure, when a dye which exhibits a high photoluminescence is selected for the light-emitting layer, high-luminance light emission can be obtained (J. J. Appl. Phys., 27, L269, 1988).
Other reports have also been announced up to the present. For example, some reports are associated with a structure of a light-emitting device obtained by combining various organic films, some report that slight light emission is recognized when a light-emitting agent is mixed with a hole-injection agent even if an organic film has a single-layer structure, and some studies are concerned with degradation of the characteristics of Alq3 serving as a light-emitting member. In addition, many applications for similar studies have been made.
A luminous intensity of an organic EL device has substantially achieved a level of practical use. However, there are many technical problems remaining unsolved in, e.g., the luminous efficiency, the service life, and the manufacturing process of the device. At present, the luminous efficiency is preferably 1%, and normally, about 0.1%. A low luminous efficiency means that a current which does not contribute to light emission flows between electrodes. Since this current generates Joule heat, it is a main cause which decreases the service life of the device. In order to put the organic EL device into practical use, therefore, luminous efficiency is preferably improved to a value from at least several % to about 10%.
In order to improve the luminous efficiency of the organic EL device, the device structure and the electrical properties of a material used for the device must be optimized. So far, the properties of the organic material have been only qualitatively defined. For example, an electron (hole) transportation property, an electron (hole) injection property, a donor type, an acceptor type, and a light-emission property have been defined. Thus, the conditions of the device have been defined insufficiently.